Method for manufacturing integrated semiconductor devices

ABSTRACT

An integrated semiconductor device is formed by bonding the conductors of one fabricated semiconductor device having a substrate to the conductors on another fabricated semiconductor device having a substrate, flowing an etch-resist in the form of a photoresist between the devices, allowing the etch-resist to dry, and removing the substrate from one of the semiconductor devices. Preferably the etch-resist is retained to impart mechanical strength to the device. More specifically, a hybrid semiconductor device is formed by bonding the conductors of one or more GaAs/AlGaAs multiple quantum well modulators to conductors on an IC chip, flowing a photoresist between the modulators and the chip, allowing the photoresist to dry, and removing the substrate from the modulator.

BACKGROUND OF THE INVENTION

This invention relates to bonding of fully-fabricated semiconductordevices onto other fully-fabricated semiconductor devices so as toproduce integrated units, and particularly to bonding fully-fabricatedphotonic elements, such as GaAs/AlGaAs multiple quantum well (MQW)modulators, onto fully-fabricated integrated circuit (IC) chips such asSi or even GaAs.

Integration of photonic devices with silicon IC chips makes it possibleto combine the advantages of each. Among photonic devices, GaAs/AlGaAsmultiple quantum well (MQW) modulators are particularly beneficial asinput/output (I/O) elements on IC chips because they have a highabsorption coefficient of light and can serve as both receivers andtransmitters. They typically operate at an optical wavelength (λ) of 850nm (nanometers).

Growing GaAs/AlGaAs on fully-fabricated IC chips has proven difficultbecause it subjects the IC chips to damage. On the other hand,techniques exist for bonding fully-fabricated semiconductor devices toeach other. However, these leave the substrates of each device in place.This subjects the bonds to adverse mechanical stresses that may affectthe devices adversely. In the case of GaAs/AlGaAs multiple quantum well(MQW) modulators, the substrates are GaAs which are opaque to theoperating wavelength of the GaAs/AlGaAs modulators, and hence requireremoval for operation. Nevertheless, it is very difficult to place andbond GaAs/AlGaAs multiple quantum well (MQW) modulators, if they havehad their substrates removed, onto silicon IC chips.

Prior techniques for bonding fully-fabricated semiconductor devices toeach other, and in particular multiple quantum well (MQW) modulators tosilicon IC chips, suffer the disadvantages of mechanical stress,opacity, or cumbersome handling.

An object of the invention is to overcome these disadvantages.

Another object of the invention is to improve bonding of semiconductordevices with each other.

Another object of the invention is to improve bonding of photonicelements with electronic elements.

SUMMARY OF THE INVENTION

According to a feature of the invention we achieve such objects bybonding the conductors of one fabricated semiconductor device having asubstrate to the conductors on another fabricated semiconductor devicehaving a substrate, flowing an etch-resist between the devices, allowingthe etch-resist to dry, and removing the substrate from one of thesemiconductor devices.

According to another feature of the invention, one semiconductor deviceis a device having one or more GaAs/AlGaAs multiple quantum wellmodulators and the other semiconductor device is an IC chip, and thesubstrate on the device with the GaAs/AlGaAs multiple quantum wellmodulator is removed.

According to another feature of the invention, the etch-resist is aphotoresist and is left to impart mechanical strength to the device.

According to another feature of the invention, the etch resist isremoved.

These and other features of the invention are pointed out in the claims.Other objects and advantages of the invention will become evident fromthe following detailed description when read in light of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a photonic device in theform of an MQW modulator containing a multiple quantum well modulatorunit.

FIG. 2 is a cross-sectional view illustrating an arrangement in a stepfor forming a device that integrates the multiple quantum well modulatorwith an integrated circuit chip according to features of the invention.

FIG. 3 is a cross-sectional view illustrating an arrangement in anotherstep for forming a device integrating the multiple quantum wellmodulator with an integrated circuit chip according to features of theinvention.

FIG. 4 is a cross-sectional view illustrating a device integrating aphotonic element with an electronic element and embodying features ofthe invention.

FIG. 5 is graph illustrating the reflectivity spectra of the MQWmodulator embodying the invention under different reverse biases.

FIG. 6 is a cross-sectional view illustrating a device integrating anumber of photonic elements on an IC and embodying features of theinvention.

FIG. 7 is a plan view illustrating a device integrating an array ofphotonic elements on an IC and embodying features of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-4 illustrate a GaAs/AlGaAs 850 nm λ multiple quantum wellmodulator MOD, and a solder-bonding technique for integrating theGaAs/AlGaAs 850 nm λ modulator with an IC to form the device embodyingthe invention. FIG. 1 illustrates a multi-strata multiple quantum wellmodulator MOD for bonding to contacts on a Si device according to theinvention.

In the modulator MOD, a GaAs substrate SUB supports a 1.5 μm layer LA1of n (i.e. n-doped) (10¹⁸ cm⁻³) Al₀.3 Ga₀.7 As grown on the substrateSUB. A 100 Å i (i.e. intrinsic) Al₀.3 Ga₀.7 As spacer SP1 on the layerLA1 spaces the latter from an i multiple quantum well modulator unit MQWcomposed of 55 periods of 90 Å GaAS wells and 30 Å Al₀.3 Ga₀.7 Asbarriers. A 70 Å i Al₀.3 Ga₀.7 As spacer SP2 on the multiple quantumwell modulator unit MQW spaces the latter from a 500 Å p (i.e. p-doped)(10¹⁸ cm⁻¹) Al_(x) Ga_(1-x) As layer LA2 graded from X=0.3 to x=0, onthe spacer SP2. A 500 Å p⁺ (5*10¹⁸ cm⁻³) GaAs layer LA3 covers the layerLA2.

The modulator MOD, at the substrate SUB, forms a 5 mm square piece andhas 110×110 μm gold p contacts CG (1000 Å thick) deposited on the layerLA3. The strata MQW, SP2, LA2, LA3, and CG form a 130×130 μm inner mesaME that extends to within 1500 Å of the n layer LA1. A 50×120 μm, 7000 Åthick, AuGe/Au n contact CO on the n layer LA1 extends upwardly to makeits top coplanar with the gold p contact CG. 4000 Å In caps coI1 andCOI2 cover respective contacts CG and CO.

Manufacture of the modulator MOD utilizes gas-source molecular beamepitaxy. The structure in FIG. 1, is manufactured according to thefollowing steps:

Growing the GaAs substrate SUB.

Growing the 1.5 μm layer LA1 of n (i.e. n-doped) (10¹⁸ cm⁻¹) Al₀.3 Ga₀.7As on the substrate SUB.

Growing the 100 Å i Al₀.3 Ga₀.7 As spacer SP1 on the layer LA1.

Growing, on the spacer SP1, the i (i.e. intrinsic) multiple quantum wellmodulator unit MQW composed of 55 periods of 90 Å GaAS wells and 30 ÅAl₀.3 Ga₀.7 As barriers.

Growing the 70 Å i Al₀.3 Ga₀.7 As spacer SP2 on the multiple quantumwell modulator unit MQW.

Growing the 500 Å p (p-doped) (10¹⁸ cm⁻³) Al_(x) Ga_(1-x) As layer LA2graded from X=0.3 to x=0, on the spacer SP2.

Growing the 500 Å p⁺ (5*10¹⁸ cm⁻³) GaAs layer LA3 on the layer LA2.

The procedure continues with:

Processing the edges to the 5 mm square piece of the modulator MOD tothe shape shown in FIG. 1.

Depositing the 110×110 μm gold p contacts CG (1000 Å thick) on the layerLA3.

Etching the 130×130 μm inner mesa ME1 around the gold contacts to within1500 Å of the n layer LA1 as shown in FIG. 2.

Deposition of the 50×120 m, 7000 Å thick, AuGe/Au n contact CO on the nlayer LA1. The contact CO is that thick in order to make its topcoplanar with the gold p contact CG.

Deposition of the 4000 Å In caps CA on both contacts CG and CO.

Etching the 200×200 μm outer mesa ME2 down into the substrate SUB.

Alloying the contacts CG and CO at 420° C. for 1 minute.

Thinning the modulator MOD to 200 μ m.

Polishing the back of the substrate SUB for viewing through an infraredmicroscope.

This completes the modulator MOD. FIG. 2 illustrates the modulator MODupside down in position above a portion of a Si device SD, such as an ICchip, as a step in formation of the integrated hybrid device embodyingthe invention. In FIG. 2 the device SD includes a 1 cm square p type Sisubstrate SIS with Al contacts COA1 and COA2 of the same size andspacing as the p and n contacts CG and CO on the modulator MOD. These Alcontacts COA1 and COA2 are set to extend out of the page of FIG. 2 sothat they would be exposed when the hybridization process is completedaccording to an embodiment of the invention. Indium contacts CI1 and CI2on the Al contacts also have the same size and spacing as the modulatorcontacts CG and CO.

To integrate the modulator MOD with an IC chip, the following occurs:

Patterning a 1 cm square p type Si substrate SIS with Al contacts COA1and COA2 of the same size and spacing as the p and n contacts CG and COon the modulator MOD. These Al contacts COA1 and COA2 are set to extendout of the page of FIG. 2 so that they would be exposed upon completionof the hybridization process according to an embodiment of theinvention.

Depositing indium contacts COI1 and COI2 on the Al with the same sizeand spacing of the modulator contacts CG and CO.

Placing the modulator MOD upside down on the Si piece and aligning it.According to an embodiment of the invention, a precision controlleraligns the modulator MOD on the Si device SD.

FIG. 3 shows the modulator MOD on the Si device SD with the In contactsCOI1 and COI2 bonded to the contacts CI1 and CI2. Here AZ4210photoresist PH surrounds the contacts CG, CO, COI1, COI2, COA1, COA2,CI1 and CI2. The structure in FIG. 3 is achieved by the following steps.

Heating the unit to 200° C. for 15 minutes to melt the indium contactsinto each other. At this point the resulting unit is relatively stable(i.e., shaking does not cause it to break apart).

Flowing AZ 4210 photoresist between the modulator MOD and the Si deviceSD by depositing drops of photoresist PH on the Si substrate about theGaAs/AlGaAs modulator MOD and allowing it to flow against its edge.

Air drying the photoresist PH for 12 hours. The dried photoresist PHserves two purposes. First, it protects the modulator MOD duringsubstrate etching. Second, it provides additional mechanical support.

FIG. 4 illustrates a structure embodying the invention. Here an ARcoating covers the MQW modulator MOD and the surrounding photoresist PH.This structure is the result of the following steps.

Placing a drop of KOH solution on the surface of the exposed GaAs toremove any GaAs oxide.

Chemically removing the GaAs substrate SUB from the modulator MOD with ajet etcher by delivering a 1×1 mm jet of etchant onto the surface of thesubstrate SUB. The etchant is 100:1 H₂ O₂ :NH₄ OH, which stops on theAl₀.3 Ga₀.7 As layer LA1. The GaAs etchant does not attack thephotoresist appreciably nor Si or Al to the sides of the GaAs/AlGaAsmodulator. However, care is taken to quickly deliver the unitarystructure into the etchant jet after applying the KOH, because KOH doesattack photoresist. The etchant etches the substrate SUB in about 1.5hours.

To prepare the integrated hybrid unit for use, the Al contacts stickingout from underneath the modulators are probed by poking the probesthrough the photoresist. These probes then provide connections to theterminals on the hybrid structure.

According to an embodiment of the invention, bond pads extend to theedge of the silicon and the photoresist is applied without coating them.According to another embodiment of the invention, the chip iswire-bonded and packaged before commencing the process.

After wire-bonding the Al pads of a modulator MOD, an SiOx AR-coating ARis deposited. The gold p contact CG serves as an integral reflector.

Yet another embodiment of the invention involves selectivephoto-chemical removal of the photoresist PH at the bond pads.

Another embodiment includes using a solvent to remove the photoresistcompletely. This leaves the integrated device of Si chip and modulatorMOD without the mechanical support of the etch resist, but also withoutthe mechanical burden of the substrate SUB.

Samples of the integrated hybrid unit have been fabricated with thephotoresist remaining on the structure. In tests made, it was possibleto completely remove the 5×5 mm substrate without damaging any section.Since the outer mesas of the modulators MOD were etched into thesubstrate the photoresist PH completely isolates the integratedstructure.

According to an embodiment of the invention, the single modulator MODand the single connection to the Si device SD of FIGS. 1 to 4 representsbut one of a number of an array of modulators MOD. Each of the latter isgrown on a single substrate and bonded to corresponding contacts on thedevice SD with the single substrate SUB then removed.

FIG. 5 shows the reflectivity spectra of a modulator MOD under differentreverse biases, measured with a lamp/monochromator. Near an opticalwavelength of 850 nm, a reflectance change from 52% to 26% occurs for a0 to 10 volt bias swing.

FIG. 6 is a cross-sectional view illustrating a device integrating anumber of photonic elements with electronic elements of an IC chip andembodying features of the invention. Here, a number of modulators MOD,identical to the modulators MOD in FIG. 4, are bonded via bondedcontacts CN collectively representing the contacts CG, CO, COI1, COI2,COA1, COA2, CI1 and CI2 to the substrate SIS of a Si device SD. Thebonding process is the same as the process in FIGS. 2 to 4 except thatall the modulators MOD start on a single substrate SUB and the Si deviceincludes a number of conductor pairs each matching the conductor pair ofthe modulator MOD above that pair. Photoresist PH extends between andaround the contacts CN and the modulators MOD. A singlepreviously-removed substrate SUB for the modulators MOD appear inphantom lines. The photoresist PH also extends between the substrate SISand the level of the removed substrate SUB.

FIG. 6 shows a single line of modulators MOD. The invention contemplatestwo dimensional arrays of such modulator MOD as shown in FIG. 7. Becauseoptical input/outputs (I/0's), such as the multiple quantum wellmodulators MOD, permit transmission and reception normal to the surfaceof the chip, such two-dimensional arrays offer substantial possibilitiesfor use in hybrid communication and information processing environments.

According to another embodiment of the invention, the photoresist PH isremoved from the structures of FIGS. 6 and 7.

In operation, the output of an off-chip laser splits into an array ofspots and focuses on the multiple quantum well modulators MOD, whosereflectance is modulated by the on-chip electronics. This type of systemoffers the advantage of having a global clock (for oscillating thelaser). In addition, it is because such modulators are also efficientdetectors that the one modulator can function as both receiver andtransmitter.

The invention furnishes a technique for solder-bonding one semiconductordevice onto another and removing the substrate from one. In particularthe invention provides a method of bonding GaAs/AlGaAs 850 nm λmodulators onto silicon. According to an embodiment of the inventionthis technique forms whole arrays of devices in one step. This techniqueprovides a method for optoelectronic integration of silicon IC's.

The invention enables the substrate of the optical GaAs/AlGaAs modulatorto be removed after it is solder-bonded to a silicon chip. Removal ofthe substrate is necessary since it is opaque to light at the wavelengthneeded for operation. In addition, substrate removal alleviatesmechanical constraints on the bond. The invention involves flowing of anetch-resist, such as a photoresist, between the chips to allow etchingof the substrate. The flow may be enhanced by capillary action. Thephotoresist protects the front sides of the chips during etching andaugments mechanical support. The technique has survived several tests ofrobustness and will support fabrication of large arrays. Althoughsimple, the invention permits the joining of complex electronic circuitswith optical inputs and outputs in large numbers.

The invention involves GaAs/AlGaAs p-i-n multiple quantum wellmodulators solder-bonded to a silicon substrate. The GaAS substrate ischemically removed to allow operation at an optical wavelength of 850nm. The gold contact to the modulator is used as the reflector. Theinvention achieves a change in reflectivity from 26% to 52% for 0 to 10volts bias swing.

The invention promotes the use of photonics in an information processingenvironment where it is integrated with electronics. The invention takesadvantage of the greater capacity of electronics for complexity,functionality, and memory, and the greater capacity of photonics forcommunications. The photonic devices, such as the multiple quantum wellmodulators, function as optical interconnects between electronicintegrated circuit chips (IC's). The invention involves the integrationof photonic elements (both receiver and transmitter) on the IC chip. Ittakes advantage of the attractive feature of optical input/output (I/0)that it can occur normal to the surface of the chip, and allowtwo-dimensional arrays of interconnects to be formed, for surface-normalphotonic operation.

The invention takes advantage of silicon electronics's effectivetechnology where complex systems such as microprocessors or memory areconcerned. It offers the benefit of increased communication capacity tothe IC chip when the chip contains a great number of computing elements.

One of the advantages of GaAs/AlGaAs multiple quantum well modulators istheir typical operation at 850 nm. This short wavelength allows theformation of small optical spots whose potential spot sizes vary withthe wavelength

The structure and process described in connection with FIGS. 1 to 4represent a single example of a integrated semiconductor device whichwas constructed and tested, and gave the results in FIG. 5. Otherembodiments of the invention use other dimensions, particularly areadimensions, and different materials. For example any suitableetch-resist, that is any polymer that resists the etchant and that driesinto a mechanically sound solid corresponding to a photoresist, maysubstitute for the photoresist AZ 4210. The term etch-resist as usedherein refers to any polymer that dries to protect an underlying solidfrom the etchant and includes a photoresist. For embodiments whichretain the etch-resist for mechanical support, a suitable etch-resistfor use herein is one that becomes sufficiently solid furnish mechanicalsupport. In embodiments which have the etch-resist removed, the etchresist need not display the supporting mechanical strength.

Moreover the contacts on each semiconductor device need not be coplanar,as long as they complement the heights of the Si-mounted contacts towhich they are to be bonded. Also, as an example, the bonding materialneed not be indium (In). According to other embodiments of theinvention, the contacts re gold, or various mixtures of In, Au, Sn,and/or Pb.

Furthermore, the Si device SD need not be a Si IC chip. The Si device SDmay be any fully-fabricated semiconductor device such as one made ofGaAs. The invention prevents the damage to the semiconductor devicewhich would be caused by growing of one device on the otherfully-fabricated device.

While embodiments of the invention have been described in detail, itwill be evident to those skilled in the art that the invention may beembodied otherwise without departing from its spirit and scope.

What is claimed is:
 1. The method of forming an integrated semiconductordevice, comprising the steps of:bonding conductors of a firstsemiconductor device having a substrate to conductors on a secondsemiconductor device having a substrate; flowing an etch-resist to filla space between the first semiconductor device and the secondsemiconductor device; allowing the etch-resist to dry; and removing thesubstrate from the second semiconductor device.
 2. The method as inclaim 1, wherein the step of flowing includes flowing the etch-resistbetween and around said conductors and around said semiconductordevices.
 3. The method as in claim 1, wherein the step of flowingincludes flowing the etch-resist between the first and secondsemiconductor devices via capillary action.
 4. The method as in claim 1,wherein the step of bonding includes forming surfaces of any one of agroup comprising In, Au, and mixtures of In, Au, Sn, and Pb on theconductors.
 5. The method as in claim 1, wherein the step of bondingincludes forming surfaces of In on the conductors.
 6. The method as inclaim 1, wherein said conductors are metallic.
 7. The method as in claim1, wherein the step of flowing includes flowing sufficient etch-resistso that said etch-resist, when dried, forms a structural support fromone of the semiconductor devices to the other.
 8. The method as in claim1, wherein the step of removing includes leaving enough etch-resist toenhance the structural support of one of the semiconductor devices tothe other.
 9. The method as in claim 1, wherein:the step of flowingincludes flowing the etch-resist between and around said semiconductordevices; and the step of removing includes leaving enough etch-resist toenhance the structural support of one of the semiconductor devices tothe other.
 10. The method as in claim 1, wherein the secondsemiconductor device includes a GaAs/AlGaAs composite material system,and the first semiconductor device includes a Si material.
 11. Themethod as in claim 1, wherein the second semiconductor device is aphotonic device and the first semiconductor device is a Si device, andsaid removing step includes removing the substrate from the photonicdevice.
 12. The method as in claim 1, wherein the second semiconductordevice includes a modulator having a GaAs/AlGaAs multiple quantum wellmodulator unit and said first semiconductor device includes a Siintegrated circuit chip.
 13. The method as in claim 1, wherein thesecond semiconductor device includes a modulator having a GaAs/AlGaAs850 nm λ multiple quantum well modulator unit and said firstsemiconductor device includes a Si integrated circuit chip.
 14. Themethod as in claim 1, wherein the step of removing includes removingsaid substrate chemically.
 15. The method as in claim 1, wherein thestep of removing includes removing oxides of said second semiconductorwith a solution before removing the substrate.
 16. The method as inclaim 15, wherein the step of removing includes removing oxides of saidsecond semiconductor with a KOH solution before removing the substrate.17. The method as in claim 1, wherein the step of removing includesremoving with a 100:1 H₂ O₂ :NH₄ OH compound.
 18. The method as in claim1, wherein said second semiconductor device includes:a GaAs portionforming said substrate of said second semiconductor device; an n-dopedAlGaAs layer; a multiple quantum well modulator unit including GaAswells and AlGaAs barriers; a p-doped AlGaAs layer; and a plurality ofgold contacts, one contacting said n-doped AlGaAs layer and onecontacting said p-doped AlGaAs layer.
 19. A method as in claim 1,wherein said flowing step includes flowing said etch-resist in the formof a photoresist.
 20. A method as in claim 1, wherein the removing stepincludes removing the etch-resist after removing the substrate from saidsecond semiconductor device.
 21. A method as in claim 1, wherein theetch resist is a photoresist, and the step of removing includesselectively removing a portion of the photoresist.